The present invention relates to a tool-driven bit, such as a router bit. More specifically, the invention concerns a bit for cutting drywall.
In the construction industry, drywall, or sheetrock, is commonly used to cover large wall areas. In a typical practice, a large drywall panel is placed over wall studs and covering typical aperture locations in the wall, such as electrical boxes, windows and doors. Once the drywall is at least temporally in place, openings at these preferred aperture locations are cut in the drywall. This approach has been found to save substantial time in comparison to precutting the panel to fit or to form a particular opening before mounting the drywall panel to the wall studs.
Special tool bits have been developed for performing such cutting operations. In particular, the bit must be capable of first axially penetrating the drywall panel, and then making a lateral cut in the panel. The typical tool, a router, operates at speeds in the neighborhood of 20,000-30,000 rpm. The drywall bit can be used first to drill through the panel directly adjacent to an electrical outlet box, for instance. The bit is then conveyed in a direction perpendicular to the length of the bit, following the contour of the outlet box. The router is then manipulated to completely encircle the outlet box to cut the preferred opening in the drywall panel.
One important factor in the design of a bit for cutting drywall is the bit diameter. Early drywall bits had a fairly large diameter, often as much as one-quarter inch (xc2xcxe2x80x3) diameter. Obviously, the larger the diameter of the bit, the wider the kerf or slot formed by the router cut. On the other hand, the larger diameter of the bit, the stronger it is, so that it will more readily resist breakage. However, the kerf width is especially important when cutting around an outlet box, for instance. Since the outlet box is ultimately covered by a cover or switch plate, it is preferable that the cut around the box be as narrow as possible. In many cases, a one-quarter inch (xc2xcxe2x80x3) slot around the perimeter of the outlet box is too large to be easily concealed by the outlet cover plate. Moreover, the width of the slot depends on the accuracy of the router operator in following the contour of the outlet box. Certainly, in some cases the slot cut in the drywall panel may move away from direct contact with the perimeter of the outlet box, which increases the area which must be overlapped by the cover plate.
In order to address this problem, drywall router bits were developed in diameters as small as one eight inch (xe2x85x9xe2x80x3). However, smaller router bits are susceptible to breakage under typical operating speeds and conditions. In order to increase the life of these smaller diameter bits, the rotational speed of the bits can be reduced. In addition, the travel speed or the cutting speed of the bit can be reduced. Of course, any reduction in cutting speed makes the drywall panel preparation process more labor intensive and ultimately more expensive.
Another problem associated with smaller diameter router bits is the noise generated by the high speed rotating bit. With many prior drywall router bits, the noise level can reach well over 100 dBA in the area occupied by the router operator. Beyond the decimal level itself, the frequency of the noise of these prior drywall bits can fall around 1500 hz, which is within the aural pain region for many drywall hangers.
Consequently, there remains a need for a drywall router bit that can optimize these strength, speed and noise considerations.
In order to address these needs, the present invention provides a router-type bit for cutting drywall or sheetrock that includes a single helical flute having a particular geometry. In one embodiment of the invention, the flute defines a cutting edge at a helix angle relative to the longitudinal axis of the bit. The helix angle is limited to a range of 36xc2x0-38xc2x0.
In another feature, the cutting edge of the flute is situated at a particular rake angle relative to the axis of the bit. In this feature, the rake angle is limited to a range of 20xc2x0-26xc2x0. In the preferred embodiment, the fluted portion of the bit has a web thickness that is between 60%-63% of the cutting diameter of the bit. In a specific embodiment, the cutting diameter of the bit is xe2x85x9xe2x80x3, or 0.0125xe2x80x3, and the web thickness is limited to 0.075xe2x80x3-0.079xe2x80x3.
In certain embodiments, the bit is configured for making an axial cut through a drywall panel. Thus, the bit includes a tip configured to perform an axial cut. In one feature of the invention, the tip includes angled chisel edges that subtend an angle of 113xc2x0-123xc2x0. Moreover, the chisel edges follow a shallow S-shaped curvature relative to a plane containing the subtended angle. Where the bit is intended for cutting around the perimeter of an electrical outlet box, for instance, the bit can include an un-fluted portion or band between the helical flute and the cutting tip of the bit.
It is one object of the present invention to provide a router-type bit for making axial and lateral cuts in drywall or sheetrock. A further object is accomplished by features that provide an optimum combination of maximized linear cutting speed, reduced noise and acceptable strength/wear characteristics.